The term “hinge for transmitting movement and/or forces” is used to mean a functional assembly suitable firstly for providing a certain amount of freedom between an upstream member and a downstream member, and secondly for transmitting either movements or forces between said two members.
In such a functional assembly, at least one upstream member such as a link is provided with a male core (e.g. spherical or cylindrical) that is received in such a manner as to be guided with small functional clearance in a concave or female bearing of a downstream member such as a clevis axis, the bearing being complementary to the core. Or vice versa if the core is downstream and the bearing upstream.
For example it can be constituted by a ball joint, a cylindrical bearing, or the like.
The concepts of movements and forces relate respectively to transmissions providing displacements of dynamic order, and those that provide zero or negligible displacements and that should be considered as being substantially static.
Depending on the type of hinge to be protected, one or more movements between the male member and the female member are possible, such as rotations (roll, pitch or yaw, for example).
The term “aggressive atmosphere” is used to mean an operating or storage environment for the hinge that is capable of spoiling or degrading the hinge, or even destroying it.
For example, an aggressive atmosphere may contain abrasive particles such as dust or sand, corrosive agents such as sea salt, or pollutants (solvent vapors, etc.), or indeed it may present physico-chemical properties that are harmful, in particular in terms of humidity, temperature (hot or cold), or radiation.
To simplify, the invention is described in the context of a spherical ball joint for the hub of the main drive rotor of a helicopter, which presents a problem of protection against abrasive atmospheres under certain conditions of use of the helicopter.
This is explained by the field from which the invention stems, however the invention is not limited in any way to this field in terms of the scope of its general aspects.
On the contrary, the invention is applicable to any other field in which it is useful to be able to protect a hinge against one or more types of aggressive atmosphere.
That said, the description returns to the above-mentioned practical example of providing protection to a hinge in the main drive rotor of a helicopter against aggressive particles that might damage it.
Under certain conditions of use, helicopters are highly exposed to aggressive atmospheres, such as dust or sand.
Certain ball joints on board such aircraft, in particular those of blade control links or of a frequency adapter (or drag damper) are thus subjected to degraded performance because of such sand or dust.
This can even be dangerous for the aircraft and for people on board and/or nearby.
It is therefore useful to protect such ball joints, which are considered as being vital components of the aircraft.
Faced with such a situation, it would appear, logical to make use of an elastomer bellows system.
Nevertheless, numerous ball joints on board helicopters are not suitable in practice for being protected merely by being covered using a bellows, in particular because of the following:                large amplitudes and high frequencies in the operation of the hinge, leading to risks of the protection breaking or suffering other damage;        high dynamic stresses applied to said hinges, leading to risks of the protection becoming separated in untimely manner from its target ball joint;        movements and forces of large magnitudes within such hinges, leading to deformations that do damage to the usual means for sealing conventional bellows;        the requirement for easy, fast, and inexpensive replacement of a protection system that has become worn, or damaged, or whenever such a replacement is required in a maintenance procedure; and        draconian standards and safety requirements which require the state of a hinge to be inspected visually on a regular basis, and thus require the protection system to be opened.        
None of the documents mentioned below describes nor suggests a solution that is acceptable in practice given the above problem.
Document FR 2 510 211 describes a hermetically sealed spherical cap gasket for a steering coupling bar of a self-propelled land vehicle. That gasket is enclosed between two compression rings. One of the rings has a bellows filled with lubricant.
A support is interposed between the rings, and is held together with them, by a metal shell, itself provided with a cap-forming dome opposite from a passage for a tail of the spherical cap.
Document FR 2 770 828 describes a pitch control device for the blades of the main rotor of a rotorcraft, the device being of the type in which each blade is firstly driven in rotation about a shaft of the rotor, and is secondly constrained in rotation with a pitch lever about a pitch-change axis of the blade, each of which levers is controlled by a pitch link.
A plate that turns with the rotor is connected to each of the pitch links by a respective ball joint at the bottom end of the pitch control link.
Document FR 2 848 524 describes a device having swashplates for controlling the pitch of the blades in a helicopter rotor. On one type of rotor that is shown, each pitch control lever is arranged to project from the leading edge of the corresponding blade via a ball joint on a sleeve connecting said blade to the hub.
A moving swashplate is also mounted on the mast and receives the ends of the pitch control levers that are remote from the blades, likewise via ball joints. The mast also receives a stationary plate secured to a structure of the helicopter via a scissors linkage whose end for connection to the stationary plate is also provided with a ball joint.
Other ball joint hinges connect the stationary plate to links for servo-controlling cyclical or collective pitch.
Document EP 0 754 623 describes an electrical connection installation between an electrical power supply conductor and a device for defrosting a rotor blade. Those blades are mounted on a hub constrained to rotate with a rotor mast and they are connected together in pairs to the hub via drag dampers. Such an interblade damper has a ball joint at each of its ends.
Document U.S. Pat. No. 6,644,671 describes a ball joint cap capable of large angle articulation for water crafts that are exposed to high levels of moisture. The cap possesses two openings in alignment, one of which includes an internal collar for attaching to the cage of the ball joint. Between the collar opening and a rod-passing opening, there is provided a bellows having special folds that can accommodate large angles.
Document WO 81/01621 describes a sealing element for a transverse fastener ball joint for a steering or transmission mechanism in a worksite land vehicle.
The elastomer element surrounds the ball joint and includes a cap. The cap possesses two transverse openings, each having a shoulder that is received in a hoop groove. Longitudinally, the cap and the element include a cylindrical closed portion.
Remote from the cylindrical portion, the cap presents a bellows from which there extends a shaft of the ball joint. The elastomer element is filled with grease.
Document WO 2004/088157 describes a sealed spherical bearing with a pair of rubber bellows rings mounted on either side of a bearing body for passing two opposite rods that are in alignment.
Each rod is integrated with a hinge ball, and close to said ball possesses a groove in which one of the bellows is assembled.